The present invention relates generally to range measuring apparatus and, more specifically, to laser range meters of the type employing a mode locked laser transmitter.
Construction and theory of operation of a mode locked laser are known, as briefly described in DeMaria, "Mode Locking Opens Door to Picosecond Pulses," Electronics, p. 112 (Sept. 16, 1968). Such lasers are capable of producing ultrashort high peak power pulses which, among other things, can be used for range measurement. It is further known that, in range measuring methods based on pulse travel time, range resolution improves with decreasing pulse duration. Nevertheless, mode locking lasers to date have not been used as transmitter oscillators in range measuring equipment primarily because ultrashort duration pulses cannot be directly utilized by relatively low speed optical/electronic components and devices available for constructing suitable receivers. For example, the time constants of even the fastest optical detectors presently available, such as avalanche or PIN photodiodes, are one to two orders of magnitude longer than the duration of a light pulse from a mode locked laser.
It is pointed out that optical correlation measurement methods using nonlinear optical elements are known, as shown by the following references:
Fricke, "Ultrakurze Laserpulse I u.II," Physik unserer Zeit, Nr. 3,84 bzw. Nr. 4,114 (July 7, 1976) PA1 Rowe, et al, "Theory of Two-Photon Measurement of Laser Output," IEEE J. Quant. Electr., QE-6, p.49 (Jan. 1970). PA1 Klauder, et al, "Correlation Effects in the Display of Picosecond Pulses by Two-Photon Techniques," "Applied Physics Letters, Vol. 13, p. 174 (Sept. 1968)" PA1 Weber, et al, "Intensity Interferometry by Two-Photon Excitation of Fluorescence," IEEE J. Quant. Electr., QE-4, p. 1009 (Dec. 1968) PA1 Moran, et al, "Interferometric Measurements of the Nonlinear Refractive Index-Relative to CS.sub.2 in Laser-System-Related Materials," IEEE J. Quant. Electr., QE-11, p. 259 (June 1975) PA1 Von der Linde, "Experimental Study of Single Picosecond Light Pulses," IEEE J. Quant. Electr., QE-8, p. 328 (Mar. 1972)
However, the described uses are for the detection and/or determination of pulse duration of ultrashort pulses, rather than the cross correlation of a measuring pulse and a reference pulse delayed in an optical delay element.
For purposes of the present invention, as well as the referenced measurement methods, an essential characteristic of nonlinear optical correlator elements is that the secondary radiation emission last several orders of magnitude longer than the primary laser pulse. This permits a radiation detector, despite its considerable time constant as compared to the laser pulse duration, to reliably convert the output signal of the optical correlator into an electrical signal.
One suitable nonlinear optical element operates on a two-photon-absorption fluorescent (TPF) effect, and utilizes fluorescent dyes such as Rhodamin 6G in ethyl alcohol solution or 1,2,3,6-dibenyanthrazen which exhibit especially good absorption characteristics for second harmonic laser light frequencies. The fluorescence yield of these dyes is proportional to the square of the light intensity because two photons of laser light are used for each atomic absorption process.
Another suitable nonlinear optical element operates on a frequency doubling or second harmonic generation (SHG) effect, and utilizes optically nonlinear crystals, such as KDP, ADP, or lithium niobate, as a laser light detector. Such crystals have polarization properties which are dependent on the square of the strength of the electrical field. As a result of the interaction with laser light, second harmonic frequency radiation is generated, with two photons of laser radiation generating one photon of secondary radiation. In addition to these methods, the use of other detection methods is also possible.
Accordingly, the present invention resides in a system concept and apparatus for a high resolution laser range meter capable of fully utilizing the extremely short pulse duration of a mode locked laser to provide improved measuring accuracy.